Photocatalytic hydrogen production from water has been pursued for over four decades now since the pioneering work of Fujishima and Honda [1]. Over five hundred different catalysts have been designed and tested; most are given in review articles [2-5]. All rely on electron excitation from the valence band to the conduction band followed by electron transfer from the conduction band to ultimately reduce hydrogen ions and to the valence band to oxidize oxygen anions. Among the many semiconductors tested is TiO2 in its different phases [6], sizes [7-9] and shapes [10-12]. While it is probably the most stable known semiconductor under reaction conditions for hydrogen production it absorbs light only in the UV (i.e. up to a maximum of about 5% of the total solar photons hitting the earth). Therefore much work has been conducted to extend its light absorption range into the visible region.
These modifications include doping with anions (such as N, S, or C anions [13])—to decrease the band gap energy by raising the VB maximum level, deposition of metals, such as Au, absorbing in the visible light (surface plasmon resonance) [14-18], as well as deposition of semiconductors that absorb in the visible light range [19, 20]. In that regard, Cu2O has an excellent potential because the position of its band gap energy falls well in the middle of the solar energy flux [21] and its band edges are appropriate for both oxidation and reduction sides of the photoreaction [23, 24]. However, it has been recognized early on that Cu cations in Cu2O are not stable. Their oxidation and reduction potentials lie in between the gap energy and, therefore, Cu+ ions can be reduced to Cu metal and oxidized to Cu2+ cations. Both (the metal and the Cu2+ cations) decrease the activity and ultimately Cu2O becomes inactive.
Previous work has attempted to change the crystal morphology of Cu2O to make it more stable. In particular certain crystal shapes have shown promise as they could withstand corrosion; i.e. depending on their crystallographic orientation. In that respect Cu2O (111) oriented surfaces have shown good activity for electron transfer (current intensity) as well as stability [24]. One of the reasons invoked is the lower resistivity values for films with a stronger (111) orientation [25] where the carrier concentration was higher by almost 2 orders of magnitude for (111)-oriented films when compared to (100); both orientations have similar charge carriers mobility.
There continues to be a need, however, for stable highly active catalytic structures suitable for use in generating hydrogen from water.